Covalent attachment of polysaccharides, that are potentially protective antigens, to carrier proteins is dependent upon both the size and structure of the two vaccine components. Methods for conjugating the Vi capsular polysaccharide, and similar polysaccharides of high molecular weight containing carboxyl functions were devised using the heterobifunctional cross-linking agent SPDP. Polysaccharides, such as the O-specific side chains of Salmonella paratyphi A and Shigella dysenteriae type 1 (Shiga) were derivatized by covalently binding the reducing terminal end with adipic acid dihydrazide using the technique of reductive amination with cyanoborohydride. The latter derivatives are coupled to the beta subunit of cholera toxin and of Shigella toxins by the carbodiimide reagent. The efficiency, physical chemical properties, standardization, and immunologic properties of these newly devised conjugates are under study.